Field of the Invention
The present invention relates to methods for identifying arcing faults within an electrical circuit having a system frequency, an electrical current and an electrical voltage, and to a circuit breaker operating according thereto.
Description of the Background Art
So-called arcing faults can occur within an electrical circuit. Arcing faults are undesired currents between at least one element of the electrical circuit and a further element, a potential difference existing between the two elements. Usually, the two elements are not in direct electrical contact, rather there is normally an insulation between them. If the insulation is defective owing to the fact that, for example, it has become porous on account of aging or has been at least partly removed on account of mechanical action, then it is possible for a further substance to penetrate between the two elements and settle there. Said substance can be liquid, for example water, or gaseous, in particular air. On account of the lack of insulation effect, in the case of the currents and/or voltages occurring within the electrical circuit it is possible for an electrical flashover, the arcing fault, to occur between the two elements.
A distinction is drawn between three different types of arcing faults, also called arc. In the case of the so-called wet arc, the penetrating substance is a liquid, usually water, which has a high electrical conductivity. The further element is usually electrically connected to ground or grounded and not necessarily part of the electrical circuit. On account of the high electrical conductivity and thus on account of the low electrical resistance of the liquid, comparatively high currents can occur between the two elements, which can lead to an impairment of possible loads of the electrical circuit.
In the case of a so-called parallel arc, the penetrating substance is a gas, in particular air. The further element is usually electrically connected to ground or grounded and not necessarily part of the electrical circuit. On account of the electric field between the two elements, the gas is ionized and an electrical current can flow between the two elements. Such a flashover is possible already at potential differences of approximately 30 volts. On account of the comparatively poor electrical conductivity of the ionized gas, the electrical current between the two elements is not constant, but rather at high frequency, which leads to a high thermal charging of the two elements, of the substance and/or of their respective environment. The temperatures occurring in this case reach as much as a few thousand degrees, with the result that further damage to the insulation, the elements and/or the respective environment is not ruled out.
In aircraft, in particular, this risk is particularly great since installed cables are usually combined to form wiring harnesses. If the first element is one of these cables, an arcing fault occurring can damage the entire wiring harness and thus jeopardize the operating safety of the aircraft. Furthermore, it is possible for elements surrounding the wiring harness or the wiring harness itself to catch fire.
A further type of arcing fault is the serial arc. In the case of the latter, the further element is the first element itself, which is usually a cable. The flashover takes place along the cable, the substance that penetrated usually being air, which is in electrical contact with the cable as a result of an insulation that has become porous. The mechanism leading to this arcing fault is comparable with the parallel arc and likewise also the effects thereof.
WO 94/22031 A1 discloses two methods for identifying arcing faults. The first of the two methods provides for monitoring the electrical current or the electrical voltage within an AC electrical circuit with regard to interference signals having a length substantially corresponding to half a system frequency.
In this case, the time periods formed between two successive interference signals are analyzed with regard to their length, their beginning or their end. If two time periods that are successive with the interval of a system frequency are present, a counter is incremented. If a specific number of regular interference-signal-free time periods is present, an arcing fault is identified. Instead of this, after registration of the specific number of arcing faults, the time periods can continue to be counted. If a time period is irregular, the counter is reduced by a predefined value. If the counter has reached a specific second number, an arcing fault is reported.
The second method involves checking interference signals with regard to their irregularities. If successive interference signals differ in terms of their position relative to the system frequency and their length, an arcing fault is identified. In addition, the rise in the respective interference signal is analyzed, since the inventor recognized that interference signals caused by arcing faults have a temporally steeper rise than possible interference signals caused by semiconductor switching elements within the electrical circuit.
The methods presented are performed by a circuit breaker, a monitoring element and a portable analysis device.
WO 2008/049436 A1 discloses a circuit breaker for monitoring loose connections in an electrical circuit. The circuit breaker trips if a high-frequency signal is detected in the electrical circuit to be monitored. In one development, the electrical circuit is interrupted by means of the circuit breaker if a specific number of such signals was determined within a predefined time window.
GB 2 348 751 A discloses a method for monitoring arcing faults within an electrical circuit realized in an aircraft. In this case, the rise of interference signals is registered and an arcing fault is identified on the basis thereof. In this case, the number of such interference signals which occur within a time window corresponding to half the system frequency is counted.